This invention relates to electrophotography and more particularly to improved electrostatographic developing materials, their manufacture and use.
The formation and development of images on the surface of photoconductor materials by electrostatic means is well known. The basic xerographic process, as taught by C. F. Carlson in U.S. Pat. No. 2,297,691, involves placing a uniform electrostatic charge on a photoconductive insulating layer, exposing the layer to a light-and-shadow image to dissipate the charge on the areas of the layer exposed to the light and developing the resulting latent electrostatic image by depositing on the image a finely-divided electroscopic material referred to in the art as "toner." The toner will normally be attracted to those areas of the layer which retain a charge, thereby forming a toner image corresponding to the latent electrostatic image. This powder image may then be transferred to a support surface such as paper. The transferred image may subsequently be permanently affixed to the support surface as by heat. Instead of latent image formation by uniformly charging the photoconductive layer and then exposing the layer to a ligh-and-shadow image, one may form the latent image by directly charging the layer in image configuration. The powder image may be fixed to the photoconductive layer if elimination of the powder image transfer step is desired. Other suitable fixing means such as solvent or overcoating treatment may be substituted for the foregoing heat fixing steps.
Several methods are known for applying the electroscopic particles to the latent electrostatic image to be developed. One development method, as disclosed by E. N. Wise in U.S. Pat. No. 2,618,552, is known as "cascade" development. In this method, a developer material comprising relatively large carrier particles having finely-divided toner particles electrostatically coated thereon is conveyed to and rolled or cascaded across the electrostatic latent image bearing surface. The composition of the carrier particles is so selected as to triboelectrically charge the toner particles to the desired polarity. As the mixture cascades or rolls across the image bearing surface, the toner particles are electrostatically deposited and secured to the charged portion of the latent image and are not deposited on the uncharged or background portions of the image. Most of the toner particles accidentally deposited in the background are removed by the rolling carrier, due apparently, to the greater electrostatic attraction between the toner and the carrier than between the toner and the discharged background. The carrier and excess toner are then recycled. This technique is extremely good for the development of line copy images.
Another method of developing electrostatic images is the "magnetic brush" process as disclosed, for example, in U.S. Pat. No. 2,874,063. In this method, a developer material containing toner and magnetic carrier particles are carried by a magnet. The magnetic field of the magnet causes alignment of the magnetic carrier into a brush-like configuration. This "magnetic brush" is engaged with the electrostatic image-bearing surface and the toner particles are drawn from the brush to the latent image by electrostatic attraction.
Still another technique for developing electrostatic latent images is the "powder cloud" process as disclosed, for example, by C. F. Carlson in U.S. Pat. No. 2,221,776. In this method, a developer material comprising electrically charged toner particles in a gaseous fluid is passed adjacent the surface bearing the latent electrostatic image. The toner particles are drawn by electrostatic attraction from the gas to the latent image. This process is particularly useful in continuous tone development.
Other development methods such as "touchdown" development, as disclosed by R. W. Gundlach in U.S. Pat. No. 3,166,432, may be used where suitable.
It is apparent that in development techniques requiring a carrier that the carrier and toner must have suitable triboelectric potential in order to properly develop the electrostatic image. The toner material for use in an electrophotographic process must be capable of accepting a charge of correct polarity when brought into rubbing contact with the surface of a carrier material, in cascade, magnetic brush or touchdown development systems. For that reason, the carrier and toner material were selected such that their triboelectric properties were different resulting in the generation of the desired charge on the toner material to enable the development of the electrostatic image.
More particularly, the toner and carrier are separated from each other in the triboelectric series, an artificial framework that positions substances according to their electrochemical potentials. Determining the triboelectric series position of a material is accomplished merely by contacting two materials, separating them and detecting the charge polarity of each with an electrometer or other suitable charge recording instrument. The series is then conventionally complied in descending order from positive to negative such that a material higher in the series charges positively with respect to those lower in the series. Although any member of the triboelectric series is itself electrically neutral, when two members (their surfaces differing in electrochemical potential) are placed in intimate contact, an imbalance in surface potential is created at their contact interface and electrons will tend to flow from the member having a lower energy level, thereby seeking to equalize the noted surface potential imbalance. When the members are separated or removed from intimate contact, the charge transfer that has occurred between such members to equalize their varying surface potentials is not given sufficient time to reverse itself and thereby retain the original electrical neutrality of each member. The net effect is an electrical surface charge present on each member, the charges being of equal magnitude but of opposite polarity. The member that is higher in the triboelectric series will have a positive polarity charge. Some resinous materials which possess many suitable properties such as fusing, transparency, adhesion, etc. for use in toners were unable to be used due to undesirable electronic properties which do not enable their use in conventional toner materials.